This invention relates to wound multipole armatures for DC motors of the type having cylindrical cases with permanent field magnets attached therein. Such motors are subject to audible noise caused by excitation of resonant case vibration modes by the alternating pattern of magnetic forces between the armature and field magnets during armature rotation.
In the case of a ten pole armature having a straight axial slot core construction, each revolution of the armature results in ten alternations of slot and pole past an axial reference line on the case. This results in a regular pattern of alternating attraction and repulsion with armature rotation. If the overall pattern excites a resonant vibrational mode of the case, the case will vibrate strongly and, in so doing, generate audible noise at a frequency which is a whole number multiple of the rotational speed of the armature. Of course, such noise depends upon the exact construction of the case and field magnets; but a cylindrical case of comparatively light construction can generally be expected to have the requisite excitable resonant vibrational modes.
Many of the type of motors under consideration have been provided with armatures having a straight skew slot and pole construction, wherein the slots and poles are skewed from the axial direction by an acute angle such as 14 degrees. Although this might be thought to alleviate the vibration problem, in many cases it accomplishes little in this area. Each axial reference line on the case still sees, on the whole, an alternation in the number of slots passing it as the armature rotates. In addition, the forces generated travel axially along the full length of the case due to the skew; and this motion excites other resonant vibrational modes of a cylindrical case structure.